A laboratory model of prostate adenocarcinoma which is hormonally responsive has long been sought as a means of identifying useful treatments. We have induced such a transplantable prostatic adenocarcinoma with 3-methylcholanthrene in androgen-treated female Praomys (mastomys) natalensis. Although the first tumors could be transplanted for only 2 generations under adverse circumstances, many other male and female animals have recently undergone prostatic carcinogen implantation and are being given various treatments (castration, testosterone, estrogen, prolactin in varying sequence and at different ages). Citric acid content and protein synthesis will be determined to test tissue sensitivity to hormone stimulation and response to treatments, supplementing measurements of tumor volume and histologic appearance. This tumor is intended to provide a model for characterizing conditions of induction, transplantation, maintenance and inhibition of prostatic cancer cell growth. The effect on established tumor of various hormones, androgen antagonists and chemo-therapeutic agents will be tested. Praomys (mastomys) natalensis has not been used heretofore in endocrine research. The structural responses of sex accessory tissue to castration and hormones have been worked out for rat and mouse and differences elucidated. We now have sufficient animals in our colony to compare the effects of androgen and estrogen on the sex accessory tissue of mastomys.